The commercial use of "privacy" windows in automobiles has become widespread. Some use is also being made of such low light transmittance windows in architectural and other fields. Finding widespread use in this respect are the so-called "dark gray" glasses or "neutral gray" glasses.
Such glasses are generally formed from a standard soda-lime-silica glass composition (e.g. as used to make conventional "float" glass) to which is added various colorants to achieve the color and spectral absorption properties desired to achieve both a pleasing color (e.g. neutral gray) and low light transmittance in the UV, visible, and IR ranges (e.g. about 300-2000 nm) Foremost among these colorants is iron, made up principally of ferric and ferrous iron. While total iron content is conventionally reported as Fe.sub.2 O.sub.3, it is recognized that ferric iron is a strong ultraviolet (UV) absorber while ferrous iron is a strong infrared (IR) absorber. The balancing of these two constituents (along with other known colorants such as Ce, Se, Ni, Co, Cr, Mn, Ti, V, Mo, and the like) has been found to be a difficult task given the need to achieve a truly commercially manufacturable and saleable product.
One such commercially acceptable product (and thus a successful balancing of constituents) is produced by Libbey-Owens-Ford Co. and is known as GALAXSEE solar privacy glass. It is believed that this glass and its composition are reported in U.S. Pat. No. 5,308,805. Therein a conventional soda-lime-silica base glass comprised, by weight percent, of:
SiO.sub.2 68-75
Na.sub.2 O 10-18
CaO 5-15
MgO 0-5
Al.sub.2 O.sub.3 0-5
K.sub.2 O 0-5
and having therein melting and refining aids such as SO.sub.3, and optionally "small amounts of BaO or B.sub.2 O.sub.3 " are provided with colorants consisting essentially of, by weight percent:
______________________________________ Fe.sub.2 O.sub.3 (total iron) 1.3-2 NiO 0.01-0.05 Co.sub.3 O.sub.4 0.02-0.04 Se 0.002-0.003 Ferrous Value 18-30% ______________________________________
The glasses of this composition are said to exhibit an excitation purity less than 10%, a dominant wavelength ranging between 490-565 nm and a color defined by the conventional CIELAB color coordinates: EQU L*=50.+-.10 EQU a*=-5.+-.5 EQU b*=0.+-.10
The patent specifically states "the presence of nickel (nickel oxides) is essential in compositions of the invention." (Col. 3, 1.48-49).
While in practice the GALAXSEE glass exhibits the necessary and desirable solar characteristics (including low UV and low total solar transmittances) to make it a commercially successful product, it has the drawback of necessitating the use of a significant amount (i.e. more than an inherent trace amount) of NiO to achieve its desirable results. NiO is known, for example, to present the potential problem of forming nickel sulfide stones in the glass.
While other patents have reported, and other commercial products have presented glasses which may be called neutral gray, low transmittance glasses, generally speaking they have been unable to achieve the full balance of characteristics which GALAXSEE has achieved, thereby rendering them less desirable in certain marketplaces. Examples of such patents (and perhaps products exemplified thereby) include: U.S. Pat. No. 5,023,210 nickel-free, gray glass of low transmittance!; U.S. Pat. No. 5,411,922 neutral gray-green nickel-free, low transmittance glasses requiring a significant amount of TiO.sub.2 to achieve its results!; and U.S. Pat. No. 5,352,640 nickel-free but requiring a high total iron content of 1.4%-4% and being, in practice, very dark in color so as to appear almost black!.
It is apparent from the above that there exists a need in the art for a new glass composition which achieves the characteristics of the GALAXSEE glasses but which is "substantially nickel-free" (i.e. contains no more than an inherent trace of nickel as an impurity; namely less than about 0.0005% by weight of the glass composition).
In addition, there also exists a need in the art for a glass composition which generically achieves low UV, low IR and low total solar (TS) transmittance values in the glass without having to use high levels (e.g. greater than 1.3% by weight) total iron (Fe.sub.2 O.sub.3) to achieve these values, regardless of whether the ultimate color achieved is a neutral gray color, or some other color that may be fully acceptable for certain other purposes in the marketplace.
It is a purpose of this invention to fulfill this and other needs in the art, more apparent to the skilled artisan, once given the following disclosure.